Here's why:

* Glycolysis: The first stage of fermentation is glycolysis, which breaks down glucose into pyruvate. This process yields 2 ATP molecules and 2 NADH molecules.

* Fermentation: The second stage uses the NADH from glycolysis to regenerate NAD+ so that glycolysis can continue. This process does not produce any additional ATP.

Therefore, the overall net gain in ATP from fermentation is 2 ATP molecules.

Important Note: While fermentation does produce a small amount of ATP, it's significantly less efficient than cellular respiration, which produces 36-38 ATP molecules per glucose molecule.